The main source of occupational exposure for workers in the nuclear industry is the dose of radiation received during routine maintenance which is performed on primary system components and equipment. The radiation level for these components results from the deposition of activated corrosion products on the interior surfaces of the system components. The corrosion products result from the release of cobalt atoms from the steel into the water system where the cobalt atoms are subjected to conditions which lead to the formation of radioactive isotope Co 59. Subsequently, these activated molecules are deposited as a film on the surface. The deposition of radioactive materials is permanent in that these activated corrosion products are incorporated into the oxide film on the pipe or vessel inside surface as a tightly adhering layer. Removal of these activated corrosion products requires chemical or physical removal techniques and cannot be performed while the plant is operating.
There is a recent trend to substantially reduce worker exposure. One method of minimizing occupational exposure has been to prevent the deposition of activated corrosion products on the interior surfaces of the water system components by protecting these surfaces with a film or coating. Several thin film methods have been studied within the nuclear industry with varying degrees of success. A reduction of the true surface area of the internal components by surface polishing has proven to be helpful. U.S. Pat. No. 4,636,266 describes a process for pre-oxidizing or passivating the highly polished internal surface. This technology is being applied to new components as they are needed in new or existing nuclear plants. Another technique has been developed by General Electric Company involving the use of zinc in the water system to inhibit the growth of films containing active cobalt corrosion products.
The present invention for the first time provides a metallic coating to reduce the level of radioactive isotope retained on the inside surfaces of water system piping and vessels. Based upon the experiments performed to date, the present invention provides a factor of ten reduction in the level of retained radioactive species.